The basic purpose of wheel alignment service is to restore any deviations on the automobile wheel/suspensions to factory specifications. These specifications apply to the vehicle when the suspension is loaded with a normal weight so that the wheels are at a normal ride height relative to a datum plane of the automobile. The ride height of the automobile is controlled by the spring tension of the suspension systems. The suspension systems distribute the weight and act as the basis for all steering geometry angles.
Apart from the wheel alignment angles (camber, caster, steering-axis-inclination (SAI) or Kingpin inclination (KPI), toe in/out, and toe-out-on-turns (TOOT)), there are the vehicle centerline, the geometrical centerline, and the thrust line. All three of these centerlines should coincide with each other.
There are several four-wheel alignment systems currently available, some of which are computerized. Examples of such systems are those manufactured by Bee-Line, Hunter, HP Andersen, Kansas Jack, and Rotary Lift. Although these systems accurately measure wheel alignment and alignment relationships between all four wheels, these measurements are not related to the vehicle itself. Thus, correct four-wheel alignment is not determined because no determination is made that the vehicle centerline coincides with the geometrical centerline which in turn coincides with the rear wheel thrust line.
With the introduction of the unibody design (self-supporting chassis) having MacPherson type suspension, front-wheel and all wheel drive, and independent rear suspensions, accurate wheel alignment has become even more critical. To make matters worse, camber and caster in many cases are not adjustable. In these cases, a very accurate chassis alignment is necessary to obtain acceptable wheel alignment values.
In U.S. Pat. No. 4,261,108 (Davis), a method for determining caster, camber, and toe-in is disclosed. According to this method, the wheel alignment is measured and adjusted without a load on the wheel and then subsequently with a load on the wheel.
In U.S. Pat. No. 3,808,695 (LaMoreux), a testing and measuring apparatus for vehicle wheel front end assemblies is disclosed. A jack is used to support the vehicle and a lever 38 is provided to impart a force onto the bottom of the tire to take up axial play in the ball joints.
In U.S. Pat. No. 4,679,327 (Fouchey et al), an apparatus and method are provided for aligning the front two planes of a motor vehicle. A pair of tie rod adjustment mechanisms including an air cylinder and drive unit are projected along an inclined path so as to enable a pair of adjustment heads to contact serrated adjustment surfaces on the tie rods. The drive pressure holds the heads in place without pushing the tie rod forward to keep from changing camber values which distort toe sets.
In U.S. Pat. No. 3,487,551 (Eggert et al), a device for indicating a condition in an unloaded state of a load carrying ball joint and suspension is provided.
A simple gage engineered to check camber, toe, and rear wheel alignment while a car is still mounted on a repair bench or anchoring system is manufactured by Steck Manufacturing, Inc. of Dayton, Ohio. The gage is in the form of a bar which is horizontally attached to the front-brake disc by magnets. Toe is checked by measuring side-to-side at each end of the bar. The bar is then attached vertically to check the camber setting using a bubble level on the bar.
It has also been disclosed that the wheels of a vehicle can be supported on individual wheel turntables during wheel alignment. However, when the front wheels are turned to the angles necessary to achieve caster measurements (usually 20.degree. in both directions), the vehicle moves sideways and thereby moves the centerline/thrust line affecting the measuring operation.